We have studied the effect of age on the increase of cytoplasmic [Ca2+] which initiates contraction in the heart muscle (the calcium transient). Right ventricular papillary muscles from the hearts of young adult and senescent rats were microinjected with the photoprotein aequorin, which emits light as a function of [Ca2+]. Tension development and intracellular Ca2+ were monitored simultaneously. The time to peak developed tension, and the half time of relaxation of tension, were longer in the muscles from the senescent animals than in the muscles from the young animals. The half time of the decline of the calcium transient, which depends on the rate at which Ca2+ is removed from the cytoplasm, was also slower in the muscles from the senescent animals. These results give more direct evidence than has been available previously that the longer contraction duration in senescent myocardium may be due to slower Ca2+ sequestration by the sarcoplasmic reticulum in the older muscle.